Maximum In Specific Heat Research Articles

Differentiated roles of Lifshitz transition on thermodynamics and superconductivity in La2-xSrxCuO4

The effect of Lifshitz transition on thermodynamics and superconductivity in hole-doped cuprates has been heavily debated but remains an open question. In particular, an observed peak of electronic specific heat is proposed to originate from fluctuations of a putative quantum critical point p* (e.g., the termination of pseudogap at zero temperature), which is close to but distinguishable from the Lifshitz transition in overdoped La-based cuprates where the Fermi surface transforms from hole-like to electron-like. Here we report an in situ angle-resolved photoemission spectroscopy study of three-dimensional Fermi surfaces in La2-xSrxCuO4 thin films (x = 0.06 to 0.35). With accurate kz dispersion quantification, the said Lifshitz transition is determined to happen within a finite range around x = 0.21. Normal state electronic specific heat, calculated from spectroscopy-derived band parameters, reveals a doping-dependent profile with a maximum at x = 0.21 that agrees with previous thermodynamic microcalorimetry measurements. The account of the specific heat maximum by underlying band structures excludes the need for additionally dominant contribution from the quantum fluctuations at p*. A d-wave superconducting gap smoothly across the Lifshitz transition demonstrates the insensitivity of superconductivity to the dramatic density of states enhancement.

Euclidean Q-balls of electronic spin/charge densities confining superconducting condensates as the origin of pseudogap and high-Tc superconducting behaviours

Euclidean Q-ball type solutions carrying Cooper/local-pair condensates inside finite volume filled with oscillating in Matsubara time semiclassical electronic spin/charge densities are found in the ‘nested’ repulsive Hubbard model of high-Tc superconductors. Euclidean Q-ball solutions, analogous to famous Q-balls of squarks in the supersymmetric standard model, arise due to global invariance of the effective theory under the phase rotation of the Fourier amplitudes of spin/charge density wave-like short-range fluctuations, leading to conservation of the ‘Noether charge’ Q along the Matsubara time axis. It is demonstrated analytically that condensation of Cooper/local pairs self-consistently creates local minimum at finite amplitude in the effective potential energy of the spin/charge density wave fluctuations. Exchange with density fluctuations of finite amplitude provides greater binding energy of fermions into local/Cooper pairs inside the Q-balls than exchange with infinitesimal lattice/charge/spin quasiparticles in the standard Fröhlich mechanism. The Q-balls arise with a finite density of superconducting condensate inside them below temperature T∗. Self-consistent solutions are found from the Eliashberg like equations with the ‘nesting’ wave vector of the spin-/charge fluctuations connecting ‘antinodal’ points of the Brillouin zone, combined with semiclassical equation for the minimum of Euclidean action of the fluctuations field. Fermionic spectral gap arises inside the Q-balls in the vicinity of the ‘antinodal’ points. The ‘breathing modes’ of the Q-balls in Matsubara time, as well as sharp maximum of specific heat across transition temperature T∗ into Q-balls gas phase are predicted.

Ising spin ladder with trimer rungs and next-nearest-neighbor coupling: frustration in physics and agent models

The extended model of two-leg Ising spin ladder with trimer rungs and next nearest neighbor interaction (NNN) in an external magnetic field is studied using the transfer matrix and linear renormalization group methods. In the standard version (with the same only nearest-neighbor interactions in both legs), such a ladder exhibits very interesting behavior—a frustration driven extremely sharp phase crossover at finite temperature that resembles a phase transition, impossible in one dimension. It is shown that in all considered cases with different interactions in each leg (asymmetric ladder), with NNN interactions and in the presence of small external field, such a crossover takes place when the point at which the effective interleg coupling vanishes coincides with the point at which the interleg correlation function exhibits an inflection point accompanied by the specific heat maximum. A hypothesis is formulated to describe an abrupt change in political view of the people where it is not necessary to resort to a concept of a phase transition, controversial for social systems. In some cases, this phenomenon can be understood as a phase crossover triggered by the level of frustration. The source of this frustration is proposed as an internal conflict between two areas of attitudes of the society members, personal and economic, which are shaped by personality traits possibly modified by life experience.

Thermal properties of epoxy nanoclay composite materials

Composite materials are an emerging topic for research as a new competitive material in engineering. New classes of composite material manufactured from particles, nanoparticles and resins, and have experienced efficient and economical for the development and also replacement of new as well as deteriorating structures. In this study epoxy-nanoclay composite materials with varying compositions of nanoclay compared with pure epoxy and epoxy with 10, 20, and 30 by weight fraction of nanoclay are prepared for better insulating materials. The various thermal properties of the material were analyzed to demonstrate that the the prepared composite is a good insulator. An increase in specific heat maximum by 11.26%, thermal stability by 58.82% results in decrease in thermal conductivity maximum by 25.65%, diffusivity to 46.8% and also co-efficient of thermal expansion with an increase in nanoclay proportion is observed. DSC, TGA and TMA are used for determining the thermal properties. SEM and EDS analysis were used to show homogeneous mixture of epoxy and nanoclay.

Unsupervised learning of topological phase transitions using the Calinski-Harabaz index

Machine learning methods have been recently applied to learning phases of matter and transitions between them. Of particular interest is the topological phase transition, such as in the XY model, which can be difficult for unsupervised learning such as the principal component analysis. Recently, authors of [Nature Physics \textbf{15},790 (2019)] employed the diffusion-map method for identifying topological order and were able to determine the BKT phase transition of the XY model, specifically via the intersection of the average cluster distance $\bar{D}$ and the within cluster dispersion $\bar\sigma$ (when the different clusters vary from separation to mixing together). However, sometimes it is not easy to find the intersection if $\bar{D}$ or $\bar{\sigma}$ does not change too much due to topological constraint. In this paper, we propose to use the Calinski-Harabaz ($ch$) index, defined roughly as the ratio $\bar D/\bar \sigma$, to determine the critical points, at which the $ch$ index reaches a maximum or minimum value, or jump sharply. We examine the $ch$ index in several statistical models, including ones that contain a BKT phase transition. For the Ising model, the peaks of the quantity $ch$ or its components are consistent with the position of the specific heat maximum. For the XY model both on the square lattices and honeycomb lattices, our results of the $ch$ index show the convergence of the peaks over a range of the parameters $\varepsilon/\varepsilon_0$ in the Gaussian kernel. We also examine the generalized XY model with $q=2$ and $q=8$ and at the value away from the pure XY limit. Our method is thus useful to both topological and non-topological phase transitions and can achieve accuracy as good as supervised learning methods previously used in these models, and may be used for searching phases from experimental data.

Specific-Heat Anomaly of the Kagomé-Lattice Ising Model in a Magnetic Field

The properties of the kagome-lattice Ising model in an external magnetic field B have not been known well. The specific-heat anomaly of the kagome-lattice Ising model in an external magnetic field is investigated based on the enthalpy fluctuation. As the magnetic field increases, the specific heat maximum decreases, but it approaches non-zero value for large magnetic field. In particular, the asymptotic line to the specific-heat maximum temperature Th(B) follows Th(B) = 0.8335565596(B + 4). The well-known experimentalist formula Tp(B) = 0.8335565596B for paramagnet shows the same slope as the asymptotic line Th (B) of the kagome-lattice Ising model in an external magnetic field.

Pyrochlore S=12 Heisenberg antiferromagnet at finite temperature

We use a combination of three computational methods to investigate the notoriously difficult frustrated three-dimensional pyrochlore $S=\frac{1}{2}$ quantum antiferromagnet, at finite temperature $T$: canonical typicality for a finite cluster of $2\ifmmode\times\else\texttimes\fi{}2\ifmmode\times\else\texttimes\fi{}2$ unit cells (i.e., 32 sites), a finite-$T$ matrix product state method on a larger cluster with 48 sites, and the numerical linked cluster expansion (NLCE) using clusters up to 25 lattice sites, including nontrivial hexagonal and octagonal loops. We calculate thermodynamic properties (energy, specific heat capacity, entropy, susceptibility, magnetization) and the static structure factor. We find a pronounced maximum in the specific heat at $T=0.57J$, which is stable across finite size clusters and converged in the series expansion. At $T\ensuremath{\approx}0.25J$ (the limit of convergence of our method), the residual entropy per spin is $0.47{k}_{B}ln2$, which is relatively large compared to other frustrated models at this temperature. We also observe a nonmonotonic dependence on $T$ of the magnetization at low magnetic fields, reflecting the dominantly nonmagnetic character of the low-energy states. A detailed comparison of our results to measurements for the $S=1$ material ${\mathrm{NaCaNi}}_{2}{\mathrm{F}}_{7}$ yields a rough agreement of the functional form of the specific heat maximum, which in turn differs from the sharper maximum of the heat capacity of the spin ice material ${\mathrm{Dy}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$.

Specific heat maximum as a signature of Mott physics in the two-dimensional Hubbard model

Recent experiments on cuprates show that as a function of doping, the normal-state specific heat sharply peaks at the doping $\delta^*$, where the pseudogap ends at low temperature. This finding is taken as the thermodynamic signature of a quantum critical point, whose nature has not yet been identified. Here we present calculations for the two-dimensional Hubbard model in the doped Mott insulator regime, which indicate that the specific heat anomaly can arise from the finite temperature critical endpoint of a first-order transition between a pseudogap phase with dominant singlet correlations and a metal. As a function of doping at the temperature of the endpoint, the specific heat diverges. Upon increasing temperature, the peak becomes broader. The diverging correlation length is associated with uniform density fluctuations. No broken symmetries are needed. These anomalies also occur at half-filling as a function of interaction strength, and are relevant for organic superconductors and ultracold atoms.

Thermal properties of coals with different metamorphic levels in air atmosphere

Thermal properties of coal are crucial in the heat-transfer process of its spontaneous combustion. Thermophysical properties of four metamorphic-grade coal samples were investigated in air using a laser-flash apparatus. Their crystallinities were analyzed by X-ray diffractometry. The results indicated that the trend in thermal diffusivity was opposite to that of crystallinity. As the temperature increased, thermal diffusivity first decreased and then increased; specific heat first increased and then decreased. With the exception of the meager lean coal sample, the trend of thermal conductivity as a function of temperature agreed with that of specific heat. As the metamorphic grade of the samples increased, the thermal diffusivity minimum and specific heat maximum shifted toward higher temperatures; in contrast, the minimum thermal conductivity shifted toward lower temperatures. From these trends, it was possible to recognize different temperature ranges according to how different properties (thermal conductivity, thermal diffusivity, specific heat, and metamorphism grade) influenced each other.

ОСОБЛИВОСТІ ЗМІНИ ТЕПЛОФІЗИЧНИХ ВЛАСТИВОСТЕЙ НАДКРИТИЧНОЇ ВОДИ ПІД ЧАС ТЕЧІЇ В КРУГЛИХ ТРУБАХ, ЩО ОБІГРІВАЮТЬСЯ

Для процесу течії надкритичної води в каналі, що обігрівається, проведено дослідження особливостей просторового розподілу фізичних властивостей води, зумовлених їх істотною температурною залежністю, характерною для надкритичних середовищ у ділянці псевдофазового переходу. На основі комп'ютерного моделювання розв'язано задачу тепломасоперенесення у вертикальній круглій трубі під час висхідного руху в ній води надкритичних параметрів. Показано, що характер розподілу властивостей теплоносія у поздовжньому перетині труби певною мірою визначається рухом в ньому фронту псевдофазового переходу. Проаналізовано вплив величини густини теплового потоку, що підводиться до стінки труби, на положення границь початку і кінця зони псевдофазового переходу і на пов'язані з цим особливості просторової зміни фізичних властивостей теплоносія. Наведено результати СFD моделювання з розподілу уздовж довжини труби, що обігрівається, таких властивостей надкритичної води, як густина, динамічна в'язкість, коефіцієнт теплопровідності і питома теплоємність. Виконано зіставлення особливостей розподілу зазначених властивостей, розрахованих за температурою на стінці труби і на її осі. Досліджено характер зміни питомої теплоємності по радіусу труби і проаналізовано вплив рівня теплового потоку, що підводиться, на положення максимуму теплоємності в різних поперечних перетинах каналу.

Origin of two maxima in specific heat in enthalpy relaxation under thermal history composed of cooling, annealing, and heating.

The origin of two maxima in specific heat observed at the higher and the lower temperatures in the glass-transition region in the heating process has been studied for polymethyl methacrylate and polyvinyl chloride using differential scanning calorimetry, and the calculation was done using the phenomenological model equation under a thermal history of the typical annealing experiment composed of cooling, annealing, and heating. The higher maximum is observed above the glass-transition temperature, and it remains almost unchanged independent of annealing time t_{a}, while the lower one is observed above an annealing temperature T_{a} and shifts toward the higher one, increasing its magnitude with t_{a}. The analysis by the phenomenological model equation proposed in order to interpret the memory effect in the glassy state clarifies that under a typical annealing history, two maxima in specific heat essentially appear. The shift of the lower maximum toward higher temperatures from above T_{a} is caused by an increase in the amount of relaxation during annealing with t_{a}. The annealing temperature and the amount of relaxation during annealing play a major role in the determination of the number of maxima in the specific heat.

On the specific heat capacity enhancement in nanofluids.

Molten salts are used as heat transfer fluids and for short-term heat energy storage in solar power plants. Experiments show that the specific heat capacity of the base salt may be significantly enhanced by adding small amounts of certain nanoparticles. This effect, which is technically interesting and economically important, is not yet understood. This paper presents a critical discussion of the existing attendant experimental literature and the phenomenological models put forward thus far. A common assumption, the existence of nanolayers surrounding the nanoparticles, which are thought to be the source of, in some cases, the large increase of a nanofluid’s specific heat capacity is criticized and a different model is proposed. The model assumes that the influence of the nanoparticles in the surrounding liquid is of long range. The attendant long-range interfacial layers may interact with each other upon increase of nanoparticle concentration. This can explain the specific heat maximum observed by different groups, for which no other theoretical explanation appears to exist.

Collinear order in the frustrated three-dimensionalspin−12antiferromagnetLi2CuW2O8

Magnetic frustration in three dimensions (3D) manifests itself in the spin-$\frac12$ insulator Li$_2$CuW$_2$O$_8$. Density-functional band-structure calculations reveal a peculiar spin lattice built of triangular planes with frustrated interplane couplings. The saturation field of 29 T contrasts with the susceptibility maximum at 8.5 K and a relatively low N\'eel temperature $T_N\simeq 3.9$ K. Magnetic order below $T_N$ is collinear with the propagation vector $(0,\frac12,0)$ and an ordered moment of 0.65(4) $\mu_B$ according to neutron diffraction data. This reduced ordered moment together with the low maximum of the magnetic specific heat ($C^{\max}/R\simeq 0.35$) pinpoint strong magnetic frustration in 3D. Collinear magnetic order suggests that quantum fluctuations play crucial role in this system, where a non-collinear spiral state would be stabilized classically.

NaCaNi2F7: A frustrated high-temperature pyrochlore antiferromagnet withS=1Ni2+

$\mathrm{NaCaN}{\mathrm{i}}_{2}{\mathrm{F}}_{7}$ is an insulating, frustrated ${A}_{2}{B}_{2}{\mathrm{F}}_{7}$ pyrochlore with magnetic $S=1\phantom{\rule{0.16em}{0ex}}\mathrm{N}{\mathrm{i}}^{2+}$ on the pyrochlore B site. Nonmagnetic Na and Ca are disordered on the A site. Magnetic susceptibility measurements made on an oriented single crystal, grown in a floating zone furnace, show isotropic behavior at temperatures between 5 and $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with an effective moment of $3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Ni}$. Despite displaying a large Curie-Weiss theta $(\ensuremath{-}129\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, spin-ordering-related features are not seen in the susceptibility or specific heat until a spin glass transition at $3.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This yields an empirical frustration index of $f=\ensuremath{-}{\ensuremath{\theta}}_{\mathrm{CW}}/{\mathrm{T}}_{\mathrm{f}}\ensuremath{\approx}36$. The spin glass behavior is substantiated by a shift of the freezing temperature with frequency in the ac susceptibility, bifurcation in the dc susceptibility, and by a broad maximum in the magnetic specific heat. The observations as made on large single crystals suggest that $\mathrm{NaCaN}{\mathrm{i}}_{2}{\mathrm{F}}_{7}$ is likely a realization of a frustrated spin 1 pyrochlore antiferromagnet with weak bond disorder.

Spin-wave theory applied to the low-temperature properties of the spin-[formula omitted] ferromagnetic chain with the y-direction exchange anisotropy

In this paper, we apply the spin-wave theory within the simple self-consistent method, and provide a simple way to investigate the spin-1/2 ferromagnetic chain with the y-direction exchange anisotropy. It is found that the anisotropy has an influence on the ground-state and low-temperature properties of the system. Anisotropy dependences of both the susceptibility maximum and specific heat maximum fit well to the exponential laws. The linear decrease is shown for the position of the susceptibility maximum. The position of the specific heat maximum displays three kinds of the anisotropy dependence, i.e., the power law, constant law and exponential law. The specific heat coefficient is calculated to be a monotonically increasing function of the anisotropy in the zero-temperature limit. At some low temperature, the Heisenberg behavior is observed to cross over the XY behavior, which agrees with the experimental result of the quasi-one-dimensional compound (C6H11NH3)CuBr3. In the isotropic and anisotropic cases, our results are in good accord with the exact Bethe-ansatz results and quantum Monte Carlo estimates, and have some advantages over the findings of Green-function method and the renormalization group method at low temperatures.

Behavior of the Widom line in critical phenomena.

Using linear scaling theory, we study the behavior of response functions extrema in the vicinity of the critical point. We investigate how the speed of convergence of the loci of response function extrema to the Widom line depends on the parameters of the linear scaling theory. We find that when the slope of the coexistence line is near zero, the line of specific heat maxima does not follow the Widom line but instead follows the coexistence line. This has relevance for the detection of liquid-liquid critical points, which can exhibit a near-horizontal coexistence line. Our theoretical predictions are confirmed by computer simulations of a family of spherically symmetric potentials.

Criticality in alternating layered Ising models. II. Exact scaling theory

Part I of this article studied the specific heats of planar alternating layered Ising models with strips of strong coupling J_{1} sandwiched between strips of weak coupling J_{2}, to illustrate qualitatively the effects of connectivity, proximity, and enhancement in analogy to those seen in extensive experiments on superfluid helium by Gasparini and co-workers. It was demonstrated graphically that finite-size scaling descriptions hold in a variety of temperature regions including in the vicinity of the two specific heat maxima. Here we provide exact theoretical analyses and asymptotics of the specific heat that support and confirm the graphical findings. Specifically, at the overall or bulk critical point, the anticipated (and always present) logarithmic singularity is shown to vanish exponentially fast as the width of the stronger strips increases.

Critical behavior of coupled4He regions near the superfluid transition

Proximity and coupling effects between contiguous regions of 4He are expected to occur over a distance on the order of the temperature-dependent correlation length ξ (t). We present recent measurements of 4He confined in arrays of (2 μm)3 boxes linked through a 4He film. The specific heat and superfluid fraction of the 4He in these geometries reveal effects which occur at distances much larger than expected. These effects include enhancements in specific heat and superfluid fraction as well as shifts in the temperatures of the superfluid onset and specific-heat maximum. Our analysis gives evidence for the relevance of the finite-size correlation length ξ (t,L) in explaining these effects even though the effects occur at distances over an order of magnitude larger than ξ (t,L). These results can be used to deduce the finite-size scaling locus for ξ (t,L). The spatial distance over which coupling and proximity effects are observed raises questions regarding the physical mechanism for these effects and the interpretation of ξ (t). Lastly, these results are discussed in connection to other measurements of 4He in confined geometries, and the relevance to other critical systems.

The field-induced laws of thermodynamic properties in the two-dimensional spin-1 ferromagnetic Heisenberg model with the exchange and single-ion anisotropies

Green's function method is applied to investigate the two-dimensional spin-1 ferromagnetic Heisenberg model with the exchange and single-ion anisotropies. In the presence of the magnetic field, the effects of the anisotropies and field on the thermodynamic properties are obtained within the random phase approximation combining with Anderson–Callen approximation. The field-induced laws are found for the thermodynamic properties. Field dependences of heights of the susceptibility maximum and specific heat maximum fit well to power laws. The linear increase at high fields is shown for positions of the susceptibility maximum and specific heat maximum. A power law at low fields occurs for the position of the susceptibility maximum. At the positions of the maxima, the magnetization and internal energy display the power-law increase and linear decrease with the field, respectively. The exponents of the power laws are dependent of the anisotropies, as well as the slopes of the linear laws. Our results do not support the 2/3 power law which was obtained by the Landau theory.

Widom line and noise-power spectral analysis of a supercritical fluid

We have performed extensive molecular dynamics simulations to study noise-power spectra of density and potential energy fluctuations of a Lennard-Jones model of a fluid in the supercritical region. Emanating from the liquid-vapor critical point, there is a locus of isobaric specific heat maxima, called the Widom line, which is often regarded as an extension of the liquid-vapor coexistence line. Our simulation results show that the noise-power spectrum of the density fluctuations on the Widom line of the liquid-vapor transition exhibits three distinct 1/f^{γ} behaviors with exponents γ=0, 1.2, and 2, depending on the frequency f. We find that the intermediate frequency region with an exponent γ∼ 1 appears as the temperature approaches the Widom temperature from above or below. On the other hand, we do not find three distinct regions of 1/f^{γ} in the power spectrum of the potential energy fluctuations on the Widom line. Furthermore, we find that the power spectra of both the density and potential energy fluctuations at low frequency have a maximum on the Widom line, suggesting that the noise power can provide an alternative signature of the Widom line.